If you want to request an autograph, follow the guidelines bellow. If you just want to mail a letter with the address above, and do not want anything back, then you can stop reading!
If you live in UK send a properly stamped and self addressed envelope (minimum size 8.5" x 4") with your request letter and a photo.
You can include a piece of cardboard to keep the photo from bending in shipping and also add "Do Not Bend" on the envelopes.
Send your letter and wait. On average, there is going to be a 3+ month wait for a response.
If you do not live in UK, you can purchase your British stamps here
*For information on postage prices to receive a letter from UK click here.

Feedback received:

Anna Acton Success! - February 4th, 2017Sent letter and photo to Anna Acton using address in database and today I received my photo back signed and personalised

Anna Acton Q+A failure, but great success - May 14th, 2015I sent a letter asking some questions, and SAE to Anna on: 22nd March 2015, and a few weeks later, i didn't get my Q+A back, instead she included a signed and personalized photo, still really happy though.
address I used was:
Anna Acton,
c/o United Agents Ltd,
12-26, Lexington Street,
London
W1F OLE
Here is the pic:

Anna Acton Sucess (Eastenders) - March 20th, 2015Sent an autograph request to Anna Acton on the 2nd January 2015 and received today a personalised signed cast card.
Took around 11 weeks to come back
Unfortunately the address I used for Anna doesn't work anymore because she doesn't work for Eastenders anymore. But I am sure her agent works just as well.